https://bugs.winehq.org/show_bug.cgi?id=48006
--- Comment #28 from Andrew Church achurch+wine@achurch.org --- I tried manually applying the ~140 patches in dlls/{jscript,mshtml} on Proton's 8.0 branch to wine-8.17, but there have been too many internal changes on Wine's side for that to go cleanly.
I suspect the most useful approach will be bisecting Proton's jscript/mshtml from 8.0 onward, and finding the particular changes that get the launcher working. I'll try and find some time for that tomorrow.